Zircon containing printing paste for overglaze coating of microelectronic circuits on ceramic substrates

ABSTRACT

Provided is an overglaze composition particularly useful for protecting a microelectronic circuit from environmental shock and contamination. Such an overglaze comprises by weight about 15-60 percent PbO, 12-40 percent B2O3, 5-45 percent ZnO and produces, upon being fired from a printed paste at a firing temperature below about 650*C., a vitreous glass coating which is substantially hermetic in its sealing characteristics. The overglaze composition may optionally accommodate up to 12 percent by weight of other various oxides such as, for example, SiO2, CuO, V2O5, BaO, Bi2O3, TiO2, ZrO2 and/or may optionally accommodate up to 25 percent by weight of dispersed particulate ceramic materials. In addition, the coating so formed, when fired upon a ceramic substrate having a coefficient of thermal expansion of less than about 100 X 10 7 in./in./*C., exhibits a tenacious bond and a structural stress at the bond interface of between about 2000 psi in compression to about 1200 psi in tension. Printing pastes are formed from these compositions by admixing the glass produced, in particulate form, with a conventional liquid organic vehicle in amounts sufficient to render the pastes printable.

tes

ietz et a1.

[11] 3,7731% [451 Jan. 22, 19741 [75] Inventors: Raymond Louis Dietz, Toledo, Ohio; Allan L. McFadden, Monroe, Conn.

[73] Assignee: Owens-Illinois, lnc., Toledo, Ohio [22] Filed: Aug. 28, 1972 [21] Appl. No.: 284,428

Related US. Application Data [62] Division of Ser. No. 84,541, Oct. 27, 1970, Pat. No.

[52] US. Cl l06/39.5, 106/47 R, 106/49, 106/53, 1l7/l25,'117/221 [51] vInt. Cl. C03c 3/22, C030 3/12, C03c 3/30 [58] Field of Search 106/47 R, 49, 53, 39.5

[56] References Cited UNITED STATES PATENTS 1,708,743 4/1929 Skarpy 106/49 2,282,540 5/1942 Deyrup 106/49 3,425,032 8/1947 Deyrup 106/49 2,468,868 5/1949 Danjin 106/53 3,061,664 10/1962 Kegg 106/53 3,250,631 5/1966 Lasher 106/47 R 3,533,861 10/1970 Foster et a1 106/53 Primary ExaminerA. B. Curtis Assistant Examiner-Mark Bell Attorney, Agent, or FirmRichard B. Dence et a1.

[ 57 ABSTRACT Provided is an overglaze composition particularly useful for protecting a microelectronic circuit from environmental shock and contamination. Such an overglaze comprises by weight about -60 percent PbO, 12-40 percent B 0 545 percent ZnO and produces, upon being fired from a printed paste at a firing temperature below about 650C, a vitreous glass coating which is substantially hermetic in its sealing characteristics. The overglaze composition may optionally accommodate up to 12 percent by weight of other various oxides such as, for example, SiO CuO, V 0 BaO, B1 0 TiO ZrO and/or may optionally accommodate up to percent by weight of dispersed particulate ceramic materials. In addition, the coating so formed, when fired upon a ceramic substrate having a coefficient of thermal expansion of less than about X 10 in./in./C., exhibits a tenacious bond and a structural stress at the bond interface of between about 2000 psi in compression to about 1200 psi in tension. Printing pastes are formed from these compositions by admixing the glass produced, in particulate form, with a conventional liquid organic vehicle in amounts sufficient to render the pastes printable.

10 Claims, No Drawings ZIRCON CONTAINING PRINTING PASTE I 01R OVERGLAZE COATING 01F MICROELECTRONIC CIRCUITS ON CERAMIC SUBSTRATES This is a division, of application Ser. No. 84,541 filed Oct. 27, 1970, now issued US. Pat. No. 3,759,727.

This invention deals with coating compositions, coatings and coated substrates produced therefrom and methods of making same. More particularly, this invention relates to overglaze compositions particularly usefulfor forming overglazes which protect and hermetically seal microelectronic circuits printed upon ceramic substrates. By hermetically seal is meant protection from contact with any substantial amount of active injurious ingredients found in the atmosphere.

The art, especially the art of microelectronic circuitry, has long known of the need to coat certain components and thereby protect them from environmental shock and contamination. For example, microelectronic circuitry printed and fired upon ceramic substrates are generally thereafter coated with an overglazing composition in paste form which, when fired at a given firing temperature, produces a glass fulfilling the described function.

As used herein, the term firing temperature means the temperature of the overglazing composition at which the viscosity of the overglazing composition is approximately poises.

Generally speaking, such overglazing compositions in paste form, must be capable of having a firing temperature below about 650C. in order to prevent undue injury to the electronic components which they are to protect, as well as for general economic reasons. One of the problems which has arisen in the art is the difficulty of achieving a paste-type of overglazing composition which will fire at the requisite low firing temperatures (below 650C. and preferably below about 600C.), and at the same time produce an overglaze which exhibits the necessary expansion and contraction characteristics to provide a tightly bonded, strongly sealed structure. While crystallized substances and borosilicates have been developed to have the necessary expansion and contaction characteristics, their firing temperatures are often very high. Furthermore, it has been found that crystallization tends to decrease the strength of the ceramic bond and increase firing temperatures. In many of the vitreous glasses developed, the requisite firing temperatures are minimized, but ad verse expansion and contraction characteristics are experienced. In some instances, the additives necessary to obtain lower firing temperatures in both the devitrified and vitreous glasses tend to be chemically reactive and thus adversely affect many of the conventional microelectronic circuits which they coat. It is therefore apparent that there exists a definite need in the art for a new glass system which may be fired in paste form at relatively low temperatures to form hermetic coatings, especially for microelectronic circuitry, which are strongly bonded and, because of their expansion and contraction characteristics, form a compatible, strong structure resistant to mechanical abrasion and thermal shock.

' lt is the purpose of this invention to fulfill the above need in the art by providing a unique overglazingcomposition which may be readly formed into a printing paste, easily printed and which possesses a firing temperature below about 650C., usually below about 600C. and, in a preferred form, below about 500C. The glass coating so formed is substantially vitreous and exhibits excellent bonding strength. In addition, the expansion and contraction characteristics are such as to provide a mechanically and thermally strong coating for overglaze purposes in the microelectronic circuitry art.

Generally speaking, the overglaze compositions contemplated by this invention comprise by weight about 15-60 percent PbO, 12-40 percent B 0 and 5-45 percent ZnO, and upon being fired when in printed paste form at a temperature below about 650C. produce substantially vitreous glass structures which are substantially impervious to injurious atmospheric contamination. Additionally, the overglaze compositions of this invention are capable of forming a strong adherent bond with a ceramic substrate and, upon being fired on said substrate, exhibit stresses at the bond interface in the coating in the range of from about 2000 psi compression to 1200 psi tension. Preferably, the coatings, when used in microelectronic circuitry as overglazes, are in a thickness of about 0.5-2.0 mils and have interfacial bond stresses in the range of from about 1000 psi tension to 1500 psi compression. Most preferably, the stresses are in compression and are in the order of about 200-1200 psi.

It has been found, especially when working with overglazes in the microelectronic circuitry art, that the above described interfacial stresses are those necessary to maintain the integrity of the system. That is to say, if upon cooling from firing there occurs at the bond interface, stresses outside of the above ranges, the coating will tend to be weakened to the point where it cannot withstand the necessary thermal and mechanical shock to which it is subjected during use in order to maintain its protective hermetically sealing qualities. While some tension stresses may be tolerated, as described, it is preferred to use glass compositions which, upon being cooled from firing, set up in compression. Such compression tends to strengthen the system as a whole and maximize protection from mechanical shock. However, if too high a compression stress is achieved, the strain upon the interfacial bond actually serves to weaken the structure while the effect upon the component hermetically sealed within is adverse.

For example, in those instances where microelectronic circuits are coated, too high a compression may actually change their electrical properties.

All of the glasses contemplated for use in this invention generally are fireable, from their paste form, at temperatures less than about 650C. However, in many preferred instances, they are fireable below about 600C. and in certain preferred instances are fireable from their paste form at temperatures less than about 500C. (i.e., about 500C. or below). Not only do such low firing temperatures optimize the economic features of processing and producing the coatings of this invention, but when such coatings are used as overglazes in microelectronic circuitry, they also insure that minimal damage will be effected to the electronic components which they seal.

Although the above-described compositions may be formulated within the above given range of ingredients, it is more preferred, especially when using the coating compositions of this invention as overglazes for hermetically coating microelectronic circuitry, to use a composition comprising by weight about -58 percent PbO, about 20-25 percent B and about 18-25 percent ZnO. A specific embodiment, particularly preferred for the purposes of this invention, for coating microelectronic circuitry within the preferred range is a glass composition comprising by weight about 57 percent PbO, about 22 percent B 0 and about 21 percent Zn().

The glass overglaze compositions contemplated by this invention may consist of a three component system of PbO, B 0 and ZnO or they may contain four, five, or more components. That is to say, this invention contemplates the consistent use of a PbO, B 0 and ZnO system within the above-described ranges which may also include other oxides which become a part of the glass structure as well. Such other oxides may be added in order to help adjust the ultimate stress achieved, lower or raise the firing temperatures, and the like for a given system and environment. Oxides of this type are well-known in the art, examples of which include SiO CuO, V 0 BaO, Bi O TiO and ZrO The alkali metal oxides such as Na O, K 0, etc., are known for their effect upon lowering the firing temperature and thus might possibly be used in some instances contemplated herein. Generally speaking, however, the glass compositions of this invention exclude the alkali metal oxides because such oxides are reactive with fired microelectronic circuitry and thus should be avoided.

The described optional oxides of the systems of this invention are usually employed, if at all, in amounts by weight not exceeding 12 percent of the total glass composition, preferably not exceeding 7 percent, and in a particularly preferred embodiment are not used at all. When the oxides are used, a particularly preferred oxide for the purposes of this invention is silicon dioxide which generally has the effect of increasing the compression stress and thus the strength of the coating.

This invention also contemplates the addition to the overglaze system of particulate adjuster materials such as particulate ceramics of conventional types. While the oxide additives described hereinabove actually become a part of the glass, the ceramic adjusters are added in particulate form and remain in substantially particulate form throughout the treatment and formation of the overglaze. That is to say, while a small amount of the ceramic material may dissolve upon firing into the glass, generally speaking, the major portion of the ceramic material added remains as dispersed discrete particles within the continuous amorphous glass phase.

The exact function of these adjusters is not known.

However, it is believed that these materials, in finely divided particulate form dispersed in the continuous vitreous phase, serve to interrupt microcrack rupture to some extent at the bond interface, but mainly, elsewhere in the overglaze system. Thus, it is believed that the use of adjusters supplements the stress at the bond interface to add strength to the system, and also serves to strengthen the other portions of the overglaze.

Generally speaking, any of the well-known ceramics may be used as adjusters for the purposes of this invention. However, the amount of ceramic added to any given system is usually limited in its upper amount by the fact that these ceramics generally raise the firing temperature of the system. Thus, for any given system, ceramic m ter als sh u d. we sljtta tewsini greater than that which would raise the firing temperature of the overglaze system above about 650C. At the same time, the upper limit of the amount of ceramic. though usually reached by the characteristic of firing temperature of the system, may, in some instances, be reached prior to this time by the addition of too much ceramic such that undue porosity exists in the system for lack of glassy binders. For most systems contemplated by this invention, the particulate ceramic material should not exceed about 25 percent by weight of the total glass system, preferably not exceeding 10 percent by weight, and in one preferred embodiment, is about 5 percent by weight of the glass system. By glass system is meant the total of the glass binder and the adjuster ceramic. In a preferred system, the ceramic material is zircon in an amount of from about 5-25 percent by weight.

Examples of ceramics which may be adjusters include the various zirconium silicates such as ZrSiO BaZrSiO MgZrSiO, and ZnZrSiO, as well as such oxides as SiO A1 0 TiO and crystallized glass particles generally of the lithia-alumina-silica type. Mixtures of these ceramics may also be employed. Usually all are employed in particle sizes averaging less than about 5.0 microns and preferably less than 1.0 micron. For the purposes of this invention, zircon (ZrSiO,) is preferred.

The sub strates contemplated for bonding the above unique overglazing composition thereto may be any of the well-known ceramic substrates conventional in the art. For best results, it is preferred to use ceramic substrates having a coefficient of thermal expansion of less than about lOO X 10 in./in./C. As hereinabove described, this invention finds particularly high utility in the microelectronic circuitry art wherein electronic circuits such as conductors, resistors, dielectrics, capacitors, and the like are bonded to a substrate in a printed predesigned pattern. Examples of the ceramic substrates conventionally used in the microelectronic circuitry art are the aluminas, the beryllias, the hafnias, cordierite ([Mg, Fe] Al Si O steatite (hydrous MgSiOQ, olivine (MgFe) SiO and mullite (3Al O 2SiO The accepted coefficient of the thermal expansion (O-300C.) for these conventional substrates are listed in the following table.

TABLE A Ceramic Substrate *a(X I0 in./in./C)

aluminas -85 beryllias about hafnia about 97 cordierite about 30 stcatites 77-82 forsterite about 99 mullite about 42 at least from O-300C.

A particularly preferred substrate contemplated for use in the environment of this invention is an alumina substrate comprised of 96 percent A1 0 which is a conventional substrate used in the microelectronic circuitry art and which has a coefficient of thermal expansion of about 79 X 10 in./in./C. A particularly preferred glass composition found to be exceptionally useful with this preferred substrate comprises by weight about 57 percent PbO, about 22 percent B 0 and sh e} luaeesnt ZnQsw iehtmaksssath w l ssy phase. Dispersed in this glassy phase is zircon (ZrSiO having an average particle size of less than about 0.5 microns. in this preferred composition, the glassy phase constitutes 95 percent by weight of the total system ally speaking, a sufficient number of layers (i.e., one or while the milled zircon constitutes about 5 percent by 5 After the overglaze paste is printed in its desired patweight of the total composition. tern, it is oven-dried at about 125C. for a period of The above-described overglaze compositions conabout 5-25 minutes and then fired in a conventional templated by this invention are particularly unique in firing furnace at the requisite firing temperature, below that they must be formulated into a printing paste, about 650C. for a period of about 4-l0 minutes at printed upon a substrate and/or components to be l0 Peak with 8-l5 minutes heat-up and cool-down perisealed and protected, thereafter fired at a temperature Otisv, U a V b l about 650C f bl below about 00 The resulting final overglaze composition is found to d i some instances b l about 50 be smooth, hermetically tight, strongly bonded to the Pastes contemplated by this invention are formulated alumina substrate and, by the Choice of the l'eqilisite by admixing an amount of particulated glass composiamount of each ingiedieht in the System, is found tion with the requisite amount of a conventional or- Provide a total Structure which is sirohgiy resistant to ganic liquid vehicle which will dissipate during an iniboth mechanical ahd thermal Shocktial drying stage and the early stages of the firing pro- The foiiowihg examples are Presented y y Ofiiius' cess and r f bl at a temperature not hi than trating further the invention hereinabove described. about 325C. Generally speaking, all of the particles EXAMPLES have a micron size of about 400 mesh or lower, and, in the instance where a ceramic adjuster is added to the The foiiowihg oveigiaze Compositions h been s stem, h glass bi d may h a i l i f l found to be useful for forming overglazes in microelecthan about-400 mesh whil th i as stated, tronic circuitry on alumina or other substrates. When tually has a particle size of less than about 5 microns formed into Overgiales, in accordance with the Printing and preferably less than about 1 micron, paste technique hereinbefore described, and fired at Any of the conventional organic vehicles well-known the c ted firing temperature upon a 96 percent aluin the printing art may be used in the practice f hi mina substrate having a coefficient of thermal expaninvention. A particularly preferred vehicle for the pursion of 79 X 10 in./in./C., the compositions of Examposes of this invention includes a mixture of butyl Car- P 1775 are subsiahtiaiiy transparent and vitreous ih bitol a etate (t d k f U i C bid C f that substantially no crystallization occurs during the diethylene glycol, monobutyl ether acetate) and isofiring P TABLE B Example 1 2 3 4 5 PbO 3o 57 50 20 B 0 30 22 30 2o 30 ZnO 40 2: IO 30 40 sio 10 10 10 a* 49 70.5 60.8 55.0 47.3 Minimum Firing Temperature C. 585C. 488C. 522C. 558C. 600C.

coefficient thermal expansion X H) in./in./C.

amyl salicylate preferably in a ratio by weight of 2:1,

preferably with a thickener such as ethyl cellulose. An especially preferred organic vehicle for the purposes of this invention consists of 95 percent by weight ofa 2:1 weight ratio of butyl Carbitol acetate and iso-amyl salicylate and 5 percent by weight of ethyl cellulose (e.g., N-ZOO).

As stated hereinabove, while the pastes contemplated by this invention may be used to coat substrates generally. in a particularly preferred use of the paste of the invention, the coating compositions are used as overglazes to protect a microelectronic circuit which has previously been coated and then fired upon a ce' ramic substrate. Generally speaking, this is accomplished by a conventional printing technique which usually consists of using a standard screen technique for printing a microelectronic circuit in a predesigned pattern upon a ceramic substrate and firing same in a known manner to produce a final circuit bonded to the substrate. Thereafter, the overglaze pastes of this invention are printed upon and over a preselected area generally encompassing the entire microelectronic circuit to be protected and a portion of the further extending substrate to which the overglaze is bonded. Gener- The glasses of Examples 1-5 were formulated in accordance with conventional techniques. Each glass was fritted and ground to an average particle size of about -4OO mesh. Thereafter, some of the fritted glasses were admixed with various amounts of zircon having an average particle size of about 0.2 microns. in the following table, the percent by weight zircon of the total glass plus zircon is indicated, the remainder being the glass frit. The so mixed glass and zircon were added to an organic vehicle consisting of 5 percent by weight ethyl cellulose (N-ZOO) and 95 percent by weight of a 2:1 weight ratio admixture of butyl Carbitol acetate and iso-amyl salicylate such that the vehicle constituted 25 percent by weight of the paste while the remainder percent by weight) was solid particulate glass plus zircon. The paste so formulated was then printed using a screen printer 200 mesh screen onto a 96 percent alumina substrate having a coefficient of thermal expansion of about 79 X 10 in./in./C. The substrate had previously been provided with a microcircuit capacitor consisting of PdAu bottom and top conductor with a capacitor dielectric material of about 1.5 mils in thickfeatures, modifications and improvements are considered, therefore, to be part of this invention, the scope of which is to be determined by the following claims:

We claim: 1. A printable paste for overglaze coating of micro electronic circuits on ceramic substrates comprising an admixture of a liquid organic vehicle which will dissi- TABLE C Ex. Glass Comp. Stress(psi) 7t Zircon Characteristics of Overglaze 6 Ex. 2 I 200 ten. clear, smooth. vitreous hermetical seal, failed thermal shock test l) 7 Ex. 2 400 ten. 25 rough, non-hermetic seal, passed thermal shock test 8 Ex. 2 480 ten. 12.5 semi-smooth, non-hermetic seal, passed thermal shock test 9 Ex. 2 350 ten. 6,25 smooth vitreous hermetic seal, strong bond, passes thermal shock test 10 Ex. 2 550 ten. 3.125 clear, smooth vitreous hermetic seal, failed thermal shock test 11 Ex. 2 800 ten. 5.0 excellent, clear, smooth vitreous seal, strong bond, passed thermal shock test 12 Ex. 3 440 ten 0 l3 Ex. 4 380 ten. 0 [4 Ex. 5 480 comp. 0 15 Ex. 1 Cracked 0 (l) Thermal shock test: dip component and overglaze into 65 C. bath and then heat to 150 C.; repeat 4 more times and observe. lf cracks occur, test is designated as a failure.

(2) The stress actually occurring is not reported. Rather, for convenience, and in accordance with conventional practice, the stress in the glaze is approximated by coating a paste bead of the composition onto the end of a glass rod which has a coefficient ofthermal expansion closely approximating the alumina substrate. In this instance, an alumina borosilicate glass rod having a coefficient of thermal expansion of about 79 x l0-"/in./in./ C. (0-300 C.) was employed. In actual matching tests, such a rod when sealed to the alumina substrate formed in tension gfl60-220 psi which is considered an acceptable match. The bead upon the glass rod-was fired at a temperature of eith er 550 C; (Ex. 6-

12) or 625 C. (Ex. 13-15) using a C./min. heat-up time. a 10 min. peak residence time and a 35C.] min. cool down time. The thicknesses of the resulting beads were approximately and respectively 6, 4, 2, 2, 2, 4, 5, 4, 3, 2 mils. The stress was measured by using a conventional polarimeter. In order to use the polarimeter. the substrate must, of course, be substantially transparent. This is the reason why the glass rod approximation test is conventional ly i ed sincg the alumina substrate is opaque.

Ofthe above examples, only examples 9 and 11 formed excellent overglazes in that they were substantially hermetic, provided sufficient bonding and strength characteristics, and passed the thermal shock test. The firing temperature used was 500C, which is very low, relatively speaking, in the art. Examples 7 and 8 may be rendered acceptable as overglazes by raising the firing temperature to about 600C. and 575C. respectively, thereby to render the coating smooth and hermetic. While example 10 did not withstand the thermal shock test when coated on alumina, this is not to say that this composition is not useful under less rigorous conditions if matched with the other substrates having different coefficients of thermal expansion within the hereinbefore described range. Example 6 illustrates the preferred composition without an additive oxide or adjuster. The composition formed a coating but because it exceeded the 1200 psi tension limit it was not wellbonded. Such an example does not, however, negate the usefulness of such a composition with other substrates and/or under less rigorous conditions. Quite to the contrary, its low minimum firing temperature makes it potentially extremely useful in areas where high temperatures cannot be tolerated.

Once given the above disclosure, many other features, modifications and improvements of this invention become apparent to the skilled artisan. Suchother pate from said paste when heated to a temperature not higher than about 325C. and a coating composition consisting essentially of about Weight Percent PbO -58 13 0: 20-25 ZnO 18-25 and about 525 percent by weight zircon, said zircon having an average particle size of less than about 5 microns, said coating composition when fired at a temperature below about 600C. being productive of a substantially vitreous glassy structure which is substantially impervious to atmospheric contamination and which is capable of forming a strong adherent bond with a ceramic substrate selected from aluminas having a coefficient of thermal expansion of about 66-85 X 10" in./in./C. and which when so fired upon said substrate exhibits a structural stress at the bond interface of between about 2000 psi in compression to about 1200 psi in tension.

2. A printable paste as defined in claim 1, wherein said structural stress is about 1500 psi in compression to about 1000 psi in tension.

3. A printable paste as defined in claim 1, wherein said structural stress is about 200 psi to about 1200 psi in compression.

4. A printable paste as defined in claim 1, wherein said coating composition consists essentially of about 57 percent by weight'PbO, about 22 percent by weight B and about 21 percent by weight ZnO.

5. A printable paste as defined in claim 1, wherein said ceramic substrate is about 96 percent A1 0 and has a thermal coefficient of expansion of about 79 X in./in./C.

6. A printable paste as defined in claim 1, wherein said particulate ceramic material is about 5 percent by weight zircon having an average particle size of less than about 0.5 micron. 7. A printable paste as defined in claim 1, wherein said coating composition is capable of being fired at a temperature below about 500C. to produce a substantially vitreous glassy structure.

8. A printable paste as defined in claim 1, wherein said coating composition includes up to 7 percent by weight of another oxide material selected from the group consisting of. CuO, V 0 BaO, Bi O T102 and ZrO and mixtures thereof.

9. A printable paste as defined in claim 1, wherein said coating composition comprises by weight about 57 percent PbO, 22 percent B 0 and 21 percent ZnO, and which at a firing temperature below about 500C. is productive of a substantially vitreous glass structure.

10. A printable paste as defined in claim 1, wherein said zircon is present in an amount of about 5 percent of said coating composition, and wherein the remainder of said coating composition consists essentially of 57 percent PbO, 22 percent B 0 and 21 percent ZnO.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT N0. 3, 787, 21.8 DATED January 22, 1974 lNVENTOR(S)': R. L. Dietz et al.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 1, line43, "contaction" should be --contraction--,-

Col. 5, line'9, "must" should be --may; Col. 8, line 58, "66-85" should be --6585-.

Signed and Seal-ed this A ttest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner ufPan nts and Trademarks 

2. A printable paste as defined in claim 1, wherein said structural stress is about 1500 psi in compression to about 1000 psi in tension.
 3. A printable paste as defined in claim 1, wherein said structural stress is about 200 psi to about 1200 psi in compression.
 4. A printable paste as defined in claim 1, wherein said coating composition consists essentially of about 57 percent by weight PbO, about 22 percent by weight B2O3 and about 21 percent by weight ZnO.
 5. A printable paste as defined in claim 1, wherein said ceramic substrate is about 96 percent Al2O3 and has a thermal coefficient of expansion of about 79 X 10 7 in./in./*C.
 6. A printable paste as defined in claim 1, wherein said particulate ceramic material is about 5 percent by weight zircon having an average particle size of less than about 0.5 micron.
 7. A printable paste as defined in claim 1, wherein said coating composition is capable of being fired at a temperature below about 500*C. to produce a substantially vitreous glassy structure.
 8. A printable paste as defined in claim 1, wherein said coating composition includes up to 7 percent by weight of another oxide material selected from the group consisting of CuO, V2O5, BaO, Bi2O3, TiO2 and ZrO2 and mixtures thereof.
 9. A printable paste as defined in claim 1, wherein said coating composition comprises by weight about 57 percent PbO, 22 percent B2O3, and 21 percent ZnO, and which at a firing temperature below about 500*C. is productive of a substantially vitreous glass structure.
 10. A printable paste as defined in claim 1, wherein said zircon is present in an amount of about 5 percent of said coating composition, and wherein the remainder of said coating composition consists essentially of 57 percent PbO, 22 percent B2O3 and 21 percent ZnO. 